Fire-retardant treatment of wood laminae

ABSTRACT

This specification describes a method conducted entirely at ambient pressure for impregnating wood laminae, particularly wood veneer intended for assembly and lamination to form plywood, with fire-retardant chemicals. Wood laminae possessing a high initial moisture content is immersed in an aqueous solution of fireretardant agent maintained at an elevated temperature for a time sufficient to achieve a desired loading, then withdrawn and stored in an atmosphere of elevated temperature and humidity for a time sufficient to permit the retained fire-retardant solution to diffuse throughout the thickness of the wood. The treated laminae is then dried and may be adhesive coated, assembled and laminated by conventional methods to form a plywood product.

United States Patent Perlus et a1.

[11] 3,840,388 Oct. 8,1974

[ FIRE-RETARDANT TREATMENT OF WOOD LAMINAE Canadian Wood Council, Ottawa, Ontario, Canada Filed: May 26, 1972 Appl. No.: 257,198

Assignee:

References Cited UNITED STATES PATENTS 8/1945 Hammond 117/59 3/1950 Loughborough et al. 117/147 X 3,158,496 11/1964 Ray et al 117/59 3,682,675 8/1972 Myers 117/116 X FOREIGN PATENTS OR APPLICATIONS 549,335 11/1942 GreatBritain ..l17/59 787,383 6/1968 Canada 156/281 VENEER HIGH MOISTURE) W291i? LOW MOISTURE) l OTHER PUBLICATIONS A Primer of Chemical Seasoning, Hill et al., pps. l-22, i 943 7 Primary Examiner--William D. Martin Assistant Examiner-Harry J. Gwinnell Y Attorney, Agent, or Firm-Burns, Doane, Swecker Mathis [5 7] ABSTRACT This specification describes a method conducted entirely at ambient pressure for impregnating wood laminae, particularly wood veneer intended for assembly and lamination to form plywood, with fire-retardant chemicals. Wood laminae possessing a high initial moisture content is immersed in an aqueous solution of fire-retardant agent maintained at an elevated temperature for a time sufficient to achieve a desired loading, then withdrawn and stored in an atmosphere of elevated temperature and humidity for a time sufficient to permit the retained fire-retardant solution to diffuse throughout the thickness of the' wood. The treated laminae is then dried and'may. be adhesive coated, assembled and laminated by conventional 7 methods to form a plywood product.

'- 22 Claims, 4 Drawing Figures un/mung? PATENTEDBBI 8 m SHEET u m a 3 840 388 x CLOSED SIDE F IRE-RETARDANT TREATMENT OF WOOD LAMINAE This invention "relates to a method of impregnating wood with chemicals by diffusion.

The method of this invention is particularly suited for impregnating with fire-retardant chemicals veneer intended for ultimate use in the manufacture of plywood. It provides a novel process for forming fire-retardant treated plywood.

BACKGROUND OF THE INVENTION Scientific and patent literature discloses processes for the impregnation of wood with 'a variety of chemicals in order to render the wood fire-retardant. Wood thus treated is' not indestructible by fire but, when submitted to high temperatures, tends to undergo carbonization with a reduced evolution of flammable gases.

The method commercially used in the past for impregnating wood'with fire-retardant chemicals, the socalled vacuum/pressure process, has proved to be expensive when applied to plywood. According to this known method, manufactured plywood, 'i.e. plywood which has been assembled from veneer and laminated into sheets, is placed in a pressure vessel and subjected to a high vacuum. A solution of the fire-retardant chemical is introduced into the pressure vessel containing the plywood sheets. Positive pressure in the order of 40-100 psi is then applied to the vessel. With some or all of the air in the pores of the wood having been removed by the initial vacuum, the effect of the positive pressure is to force the fire-retardant solution to enter the voids. The impregnated plywood is then dried under mild conditions in a kiln. v

The product of this treatment technique is generally good from the standpoint of fire-retardation..I-Iowever, a substantial drawback of this'process in its application to plywood lies in the-high costof the-treatment, which is related both to the substantial time required for treament and to the fact that the treatment is essentially a batch, not a continuous, process. This high cost of treatment is reflected in the relatively high cost of present fire-retardant treated plywood.

A plywood processed as a laminate is a relatively thick material and substantial time is required in the existing process to achieve satisfactory impregnation. For

example, the pressure treatment of three-fourths inch Douglas fir plywood may require as long as 6 to 8 hours in the pressure vessel. Following the impregnation, the product will contain the solvent for the fire-retardant chemical, in most cases water, in the range of 100 to 130 percent of the OD (oven dried) weight of the wood. The time required to remove this volume of water is considerable. It has been found that the time required to dry the treated plywood increases exponentially as the thickness of the sample treated increases.

' The drying must proceed under mild drying conditions to avoid warping or checking or cracking of the wood. At present, as long a period as fourteen days is required for the impregnation and drying of three-fourths inch plywood by this conventional method.

It follows that with this treatment expensive vacuum/pressure equipment is tied down for relatively-long periods of time per unit of material treated. Further, such equipment requires operators of a higher than ordinary skill, as well as safety supervision. These factors add to the cost of the product.

Increased costs may also be incurred because, in the drying step, the large amount of water withdrawn per unit of surface area results in a proportionally large amount of the fire-retardant agent being carried. to the surface of the product. Not only does this lead to a reduced retention of the fire-retardant agent but where efflorescence results, additional sanding of the dried product may be required to produce a commercially attractive product. I

As a result, there has existed a need for a method of manufacturing fire-retardant plywood which is more economical in terms of time of treatment and investment in process equipment, and which may be adapted to continuous operation, to allow the productionof a lower cost product of satisfactory flame spread rating without loss of other desirable properties.

SUMMARY OF THE INVENTION It has been found that these ends may be accomplished by a method conducted entirely at ambient pressure in which wood veneer possessing a high initial moisture content is immersed in a solution of fireretardant agent and is thereafter stored for a period of time in an atmosphere of elevated temperature and humidity. After the fire-retardant agent diffused through the veneer, the veneer is dried. A plurality of veneers so treated may then be laminated by the application of adhesives, assembly of the veneers and hot or cold pressing in the known manner.

DESCRIPTION OF DRAWINGS treatment is plotted for a number of layers through a sample of treated veneer.

With particular reference to FIG. 1, it will be seen that in the method of this invention, wood laminae 1, which may be green veneer taken directly from a log by rotary peeling, or veneer from storage, is presoaked in a bath of water 2 maintained at elevated temperature,

to increase the moisture content of the wood-to about,

percent OD weight or greater. Veneers having the required moisture content are immersed in a bath 3 of concentrated fire-retardant solution for a time sufficient to achieve a pick-up to the required level. Where the veneer to be treated has a sufficiently high initial moisture content, it will be possible to omit this initial presoak step, as schematically shown by the direct transfer la to the bath of fire-retardant solution 3-without'the presoaking step. It has been found that after the immersion in bath 3 of the method, the distribution of the fire-retardant agent tends to be uneven. The diffusion of the fire-retardant agent throughout the lamina to provide a more even distribution is achieved by dead storing the veneer in anatmosphere of elevated temperature and humidity, schematically indicated by the reference character 4. The impregnated veneers are 3 then dried'in a conventional ovenand subsequently, where the desired product is a plywood, assembled with the application of adhesive and subjected to. press cur-- North America and one of the more difficult species of H6 and H10 inch were selected for experimentaluse,

the former thickness being considered typical of the thickest encountered in commercial use. A reduction in thickness, say from l/6 to l/lO in., makes a substantial difference in the ease with which the lamina can be impregnated.

. The moisture content of most veneers which will be treated by the method of this invention will be below the optimum level specified herein. Moreover, because any single lamina may have a moisture content which is not constant throughout the lamina, it'may be desirable to equalize the moisture level of the lamina by presoaking in water. However it should be made clear that presoaking can be eliminated entirely in the case of veneers having a sufficiently high and even moisture content.

.For any given wood lamina, the increase in moisture content on presoaking isa function of temperature and time. The moisture absorbed increases with an increase in temperature. At a given temperature, the moisture content tends toward a'maximumattainable level with time. For Douglas firheartwood veneer, it was found that 100 percent OD weight moisture content could be achieved by a presoak for 24-48 hours in water at 80 The initial moisture content of the lamina may vary from about 10 to, -1 10 percent OD weight. For example, a veneer of Douglas fir, heartwood will generally have an initial moisture content of about 35 to 40 percent OD weight. v

It has been determined that' the amount of fireretardant chemical absorbed by a wood lamina after a period'of immersion is influenced to a large-degree by the initial moisture content of thelamina. FIG. 2 is a curve which reflects experimental results when a series of 3 5 /2 infpanels of Douglas fir veneer of H6 in. thickness, of variable moisture content, were immersed for 24 hours in a saturated solution of monoammonium phosphate, a widely used fire-retardant chemical.

For high pick-up of the salt, a high moisture content in the wood laminae is required. A moisture content of 70 percent OD weight or greater, and preferably about I00 percent'OD weight or greater has been found to result in a loading of the lamina sufficient to yield a sat- 4 isfactory product in the practice of this method. Further, if a lamina of lower moisture'content is used, unevendistribution of the impregnating-agent often occurs. The result is a lowering of the fire-retardant efficiency which would be indicated, for example, by'a higher flame spread rating when measured by standard test methods.

' Veneer having the desired moisture content is immersed in a hot solution of the fire-retardant agent.

Many chemicals are known to be useful as fireretardant agents and may be used in the method of this invention. Among such agents are water soluble organic salts and water soluble inorganic salts and mixtures thereof. The water soluble inorganic salts are most commonly used in fire-retardant treatment of wood. These include ammonium sulfate, mono and diammonium phosphate, sodium borate, boraxboric acid mixtures, ammonium sulfamate, chromated zinc chloride, ammonium polyphosphate fertilizer and high phosphate fertilizers (e.g. with nutrient contents of l 1% N, 37% P 0 0% K 0 or 18%N, 46% P 0 0% K 0), among others. The agent selected for use with the present method should be one with a high solubility in water and further, a solubilitythat is highly temperature dependent, so that adequate loading can be attained with a relatively brief soaking in a hot saturated solution. Investigations to determine the-most economical fire-retardant agent have shown that monoammonium phosphate is particularly preferred for the method of this invention. Though this salt is subject to leaching, its use in the present method is preferred because of its present relatively low cost per pound, its efficiency as. a flame retardant and desirable afterglow inhibition properties, its low hygroscopicity, its high stability and its solubility at elevated temperatures; the latter property allows a high salt loading in the volume of liquid penetrating the wood. It also has the advantage that corrosion problems with steel are slight especially when a buffering agent is used. If necessary, corpercent salt pick-up.

rosion inhibitors may be added to the solution'of fire retardant agent. Although for some fire-retardant agents and/or laminae of wood having high absorption characteristics, an'uptake of as little as 2lbs. per cubic foot of wood yields a product of adequate flame spread rating, it has been found that for most fire-retardant salts, a salt pick-up of 5-6 pounds of salt per cubic foot of wood is adequate to provide a flame spread rating of 25, (when tested by ASTM method E-84). In the case of Douglas fir which has a density of approximately 30-34 lbs/cu. ft. this is equivalent to about a 15 20 For maximum pick-up, it is desirable that the solution of fire-retardant salt be a saturated solution. The concentration of salt in such a solution will of course be governed'by temperature of the bath. Table I shows the relaiion between salt pick-up and variations in bath temperature. The bath contains a saturatedsolution of monoammonium phosphate. The period of immersion of samples of Douglas fir veneer was 5 minutes for l/lO in. thick panels and 9 minutes for H6 in. thick panels.

Table 1 Lamina Salt Solution Lamina Salt Thickness (g/100 g H O Temp.(C) Moisture Pickup As appears from Table I the variation in salt pick-up is greater than would be expected from the magnitude of the differences in salt concentration in themselves. This is attributable to the variation in temperature. Raising the temperature (and thus the saturated salt concentration) markedly increases the salt pick-up. For thinner veneers, the temperature of the bath is a less important factor since penetration is relatively fast and easy, but in general, a solution temperature of 7595C is preferred in the practice of the invention to achieve the highest loading.

Comparative tests have been carried out to assess the salt pick-up of lamina of various species of wood. The laminae tested varied as to moisture level but the results were computer analyzed to adjust the figures to a common calculated base of 100 percent OD weight. The salt was monoammonium phosphate. The temperature conditions were uniform. The test results are summarized in Table II.

Table II Salt Pick-up of Various Species Salt Pick-up (100% OD weight) The length of the period of immersion which can be varied as convenient to balance out the effect of other variables as the permeability of the wood, the moisture content of the lamina, the bath temperature, etc., to achieve the required salt loading. An immersion period from 3-15 minutes will ordinarily serve to produce the desired loading, with the preferred time period being about 9-12 minutes.

Upon cooling, after removal of the lamina from the salt solution bath, crystals of salt form at or near the surface of the lamina when it cools. At this stage, the salt'concentration near the surface of the lamina is high. Although some salt can also be found at the centre of the lamina, the concentration there is low, usution, samples of 5 /2 X 3 inch Douglas fir veneers ob-' ally less than 3 percent. This salt has probably entered through lathe checks rather than by cell to cell diffusion. 1f the lamina were allowed to remain in the solution for a long time an even distribution of salt would be obtained. However, because of the cost of maintaining the salt solution at a high temperature, it is desirable for reasons of economy that the laminae remain in the solution for as short a time as possible.

To obtain even distribution of the fire-retardant agent throughout the wood, the diffusion is permitted to continue through a storage period, during which the laminae are deadpiled in an atomsphere of elevated temperature and humidity. The temperature maintained may suitably be in the range of from room temperature to about 97C with a preferred range of approximately 40-70C. The humidity is maintained above the equilibrium relative humidity of the salt or salt mixture employed and preferably as close to 100 percent RH. as possible. During storage under these conditions, the crystals are slowly redissolved and the dissolved salt in solution diffuses to the centre of the lamina. The slightly elevated temperature speeds up the diffusion process and helps to keep the salt in solution. The high humidity is required to prevent the :laminae from drying out and to conserve water for the diffusion function. Therefore in order to obtain an even salt distribution, the storage period should be maximized. An adequate salt distribution may be obtained using a storage (diffusion) period of between 1 to 4 days depending on the thickness of the lamina treated.

After deadpiling the laminae are dried to remove the residual water in which the impregnating'salts are dissolved. In order to prevent salt migration to the surface, this drying should be carried out under mild conditions. The drying may be carried out either by air drying-or by drying at an elevated temperature. However, the temperature at which drying is carried out alsoaffects the distribution of the salt. Air drying the veneer at room temperature results in a good distribution of fireretardant salt in the veneer. However it has been found that a drying temperature of up to 98C provides a satisfactory product. Above 98C, salt tends to be redistributed toward the surface in the drying operation. Any excess agent at the surface of the veneers may be removed.

EXAMPLE 1 To ascertain the general pattern of salt distribution in veneer treated according to the method of this inventained froma variety of commercial sources were treated. Samples of 1/6 in. thickness were chosen because they are representative of one of the thickest .used in industry. The veneer varied in its'initial moisture content but was raised to a suitable level by presoaking. The samples were impregnated by immersion in an aqueous solution of monoammonium phosphate saturated maintained at C for 48 hours. At' this temperature, the bath contained l30gms. of salt per 100 gms. of water (56.5 gms. salt per l00 gms. of solution).

After dipping, the veneer was then deadpiled in con- Lot I Lot 2 Lot 3 Lot 4 Average moisture content before dipping(l%) Il3 I13 I13 I13 Salt solution temperature (saturation temp.) (C) 85 85 85 85 Dip time (minutes) I2 I2 l2 I2 Deadpile time (hours) 96 96 96 Deadpile temperature (C) 70 70 23 Deadpile humidity RH) -IO -l00 -l00 Drying temperature and time 23C at 160C for 20 50% RH minutes then for 4 105C for 4 days hours To ermine. p s hats. .dis ri t q Slabmple were cut from each veneer of a size 1 X A in. These sub-samples were placed in a microtome holder designed to permit slicing of the sub-sample parallel to the veneer surface. Ten slices were cut from both open and closed faces of the veneer. The first three or four slices, depending on the surface roughness were cut to a nominal thickness of 100 pm; the subsequent six or seven slices were each 300 am.

To minimize the effect of heterogeneities some to sub-samples were taken and analysis performed on the combined slices all cut from the same depth.

The combined slices were extracted by boiling for one-half hour periods with three separate 75 ml. portions of distilled water. The extracts were combined and diluted to 1 liter Aliquots ranging from 1 to 5 ml., depending on the phosphate concentration, were then taken and placed in a 50 ml. volumetric flask. To this was added 20 ml. of water and 3 ml. of Lucena-Conda- Pratt reagent prepared according to theprocedure described in Anal. Chim. Acta, 16,473 (1957). The flasks were heated over boiling water. for 1 hour, cooled, and the contents diluted to 50 ml. The absorbance is then measured at 830 am on a spectrophotometer and the phosphate content calculated usinga calibrated curve obtained from a standard solution.

It was determined that the distribution pattern under the test conditions takes the form of a distorted parabola (see for example FIG. 4). The minimum of salt was found at a depth approximately 1,000 to 2,300 am from the closed face. The theoretical geometric centre of the veneer was about 2,100 am from the face. The distortion in the parabola reflects the increased permeability of the veneer from the open face, primarily due to lathe checks. Lathe checks were found to vary in size and number even within one batch of veneenWith this in mind, the analysis was made from the closed face, since it was considered that the closed side of the sample presented a more reliable substrate for experimental work.

The following Table III and FIG. 3 summarizes distribution found in veneers at various stages of the treatment ofthis invention.

Table III Phosphate (PO,---) as NH H PO Lot 2 .Lot 3 Lot 1 0 The marked contrast between the distribution in Lots 1 and 2 illustrates the importance of the deadpiling step in obtaining even distribution. A comparison of Lots 3 and 4 shows that when the drying step is carried out at high temperature, there is some migration of salt to the surface of the dried veneer.

EXAMPLE 2 Example 1 was conducted with laminae having a very high average moisture content of 1 13 percent, obtained by soaking air dry veneers for 48 hours at C. The result was an adequate loading and proper distribution of fire-retardant salt in the treated laminae. The importance of the high moisture content in the laminae to be immersed may be illustrated by the results of the following example in which the same analytical procedure was followed after the treatment of samples of veneer of similar properties under the following conditions:

Average moisture content before dipping 89.9 91.0 Salt solution temperature (saturation temp.)(C) 85 85 Dip time (minutes) 5 5 Deadpile time (hours) 8 8 Deadpile temperature 50 40 Dea'dpilc humidity RH) -I00 -l00 Drying temperature (C) 95 Drying time To constant To constant weight weight 15.4 14.3

Average salt retention The following Table IV and FIG. 4 indicate distribution of salt in veneer treated under such conditions. It will be seen that less salt has reached the centre of the laminae and distribution is uneven.

Table IV Salt distribution after diffusion treatment Salt distribution after diffusion treatment Layer Depth of Pene- Percent phosphate No. tration (um) Po as NH.H PO

Closed Side Open Side Experiment B As noted above, the distribution pattern of FIG. 4 is a parabola distorted toward the closed face of the sample. It was found that if the two sides of the parabola are separately analyzed between the outer face and the depth at which the salt concentration was minimum, the distribution could be described by the equation:

where Y salt concentration at layer x A the theoretical salt concentration at the surface of the veneer or at zero depth natural logarithm B apparent diffusion resistance x depth of layer x from the surface (in mm.)

For an ideal saturation of perfect diffusion B is zero and A would be the salt concentration throughout the veneer. Thus, the larger the value of B the more uneven the salt distribution.

Constant B is always positive, because the value of Y decreases from the value of A. The value of B characterizes the degree of decrease. If there were node crease in salt concentration throughout the veneer B would equal 0. 1t has been'found that, generally speaking, that with an increase in the time of deadpiling, the B value approaches 0. An increase in diffusion dwell temperature gives inconsistent results but it does not appear likely that diffusion dwell temperature has a significant influence on B.

This equation may be used to optimize the treatment of any particular species of wood veneer to provide high salt loadings coupled with even distribution through the veneer.

While in the foregoing examples, the fire-retardant agent employed was momoammonium phosphate, it is stressed that the present invention is in no way limited to this material and otheragents may be employed in the method.

EXAMPLE 3 which different samples were treated to immersion periods in the boric acid/borax solution of 3, 6, 9 and 12 minutes respectively. All samples were then deadpiled for 16 hours at 60C under conditions of approximately 100 percent (RH). The results in terms of average salt retention are set forth in Table V as follows:

Dip Time (Minutes) Salt Retention Laminae treated with fireproofing agents in accordance with the present invention may be used to manufacture plywood using a conventional hot or cold press method. In forming plywood in accordance with such methods it was found that melamine fortified urea resin or resorcinol glues provided satisfactory gluelines.

Tests were also conducted to determine suitable waterproof glues compatible with treated veneers. While it is possible to glue with commercially available phon'olic paper glueline, we have found that phenolic resin glues of the following formulas were particularly suitable aswaterproof adhesives for use in gluing Douglas fir veneers impregnated with-momoammonium phosphate:

Formula 1 BRL 1215* 59% Cellulosic Filler 5% Wheat Flour 10% Water 26% Glue Spread 38 to 40 lb.

Formula 2 BRL 1216* 84% Cellulosic Filler 11% Water 5% Glue Spread 40 lb.

Formula 3 BRL1216* Cellulosic Filler 12% Water Glue Spread 40 lb.

Because of the interaction'between thecausticglues such as liquid phenolic resins and the acidic fireretardant salts, long assembly times are detrimental to the bond, though favoured in industrial practice. It has also been found that the moisturecontent off the ve neer should be reduced to a very low level (eg. less than 1 percent) prior to gluing in order to reduce the movement of the saltto the veneer surface with residual water during pressing.

Although high pressing temperatures are desirable for rapid curing of the glue, the higher the temperature the greater the discolouration and degradation of the treated veneer. A pressing temperature of 280F and a pressure of 200 psi was found to produce a satisfactory plywood from fire-retardant treated Douglas fir veneer bond by the liquid phenolic resin Formulae l -3 referred to above.

wood desirable for special applications.

The present method provides a means to producetreated laminae and plywood by a mass producing method. Because the method is performed at atmospheric pressure, the vacuum/pressure equipment dictating batch treatment is avoided and the cost of the product lowered.

From the foregoing description it will be seen that the novel method provides a simple and economical means to provide fire-retardant plywood or other laminated products at a commercially attractive price.

While the invention has been described specifically herein, various changes and modifications may be made without departing from the spirit and scope of the invention, as will be apparent to those skilled in the art to which it pertains. It is accordingly intended that the present invention shall only be limited by the scope of the appended claims.

We claim:

1. A method for impregnating wood laminae which comprises: v

selecting wood laminae having a minimum moisture content of about 70 percent oven dried weight, subjecting said laminae having a minimum moisture content of about 70 percent oven dried weight to a treatment conducted at ambient pressure which comprises successively:

i. immersing said laminae in a concentrated aqueous solution of fire-retardant agent maintained at an elevated temperature of about 75 to about 95C., for a time sufficient to achieve a loading of the laminae with said flame retardant agent in an amount of at least 2 lbs. per cubic foot,

ii. withdrawing said laminae from said solution and storing them in an atmosphere at a temperature .in the range from room temperature to about 97C. maintained above the equilibrium relative humidity of said'aqueous solution for a time sufficient to permit solution retained by each lamina to diffuse throughout its thickness, and

iii. drying said laminae.

2. The method as defined in claim 1 in which the selected wood laminae have a minimum moisture content of about 100 percent oven dried weight.

3. The method as defined in claim 2 in which the minimum moisture content is attained by pre-soaking said laminae in water at an elevated temperature fora time sufficient to produce the minimum moisture content desired.

4. The method as defined in claim 1 in which said immersion in the solution of fire-retardant agent is for a time sufficient to achieve 'an uptake of said flameretardant agent of at least 5 pounds per cubic foot of wood.

5. The method as defined inclaim 1 in which said fire-retardant agent is a water soluble organic salt, a water soluble inorganic salt, or mixtures of two or more of said salts.

6. The method as defined inclaim 2 in which said fire-retardant agent is an inorganicsalt'having a high solubility in water and a solubility which is highly temperature dependent, or a mixture of two or more of such salts.

7. The method as defined in claim 2 in which said solution of fire-retardant agent is a saturated solution of ammonium sulfate, monoammonium phosphate, diammonium phosphate, sodium borate, borax-boric acid mixtures, ammonium sulfamate, chromated zinc chloride, an ammonium polyphosphate fertilizer, a high phosphate fertilizer, or mixtures thereof.

8. The method as defined in claim 1 in which the laminae are immersed in said solution for a period of from about 3 to 15 minutes.

9. The method as defined in claim 2 in which said solution of fire-retardant agent is a saturated solution of monoammonium phosphate and said laminae are immersed in said solution for a period of approximately 9 to 12 minutes.

10. The method as defined in claim 1 in which said laminae are stored in an atmosphere of about 100 percent relative humidity for a period of from 1 to 4 days.

librium relative humidity of said aqueous solution at a temperature in the range from about. 40C. to about C. for a periodfrom l to 4 days, and drying said laminae.

12. In a process for manufacturing fire-retardant plywood from a plurality of wood veneers, the improvement which consists of subjecting veneers selected to have a minimum moisture content of about 70 percent oven dried weight before adhesive coati g, assembly and lamination, to a treatment conducted at ambient pressure which comprises successively:

i.- immersing said veneers in a concentrated aqueous solution of fire-retardant agent maintained at an elevated temperature of about to C., for a time sufficientto achieve a loading of the lamina with said flame-retardant agent in an amount of at least 2 lbs. per cubic foot,

ii. withdrawing said veneers from said solution and storing them in an atmosphere at a temperature in the range from room temperature to about 97C. maintained above the equilibrium relative humidity of said aqueous solution for a time sufficient to permit solution retained by each lamina to diffuse throughout its thickness, and

iii. drying said veneers.

13. Ina process for manufacturing fire-retardant plywood from a plurality of wood veneers, the improvement which consists of subjecting said veneers before adhesive coating, assembly and lamination to a treatment conducted at ambient pressure which comprises successively:

i. presoaking said veneers in water at an elevated temperature for a time sufficient for the veneers to attain a minimum moisture content of about 70 percent oven dried weight,

ii. immersing said veneers in a concentrated aqueous solution of fire-retardant agent maintained at an elevated temperature of about 75 to about 95C., for a time sufficient to achieve a loading of the lamina with said flame-retardant agent in an amount of at least 2 lbs. per cubic foot, 1

iii. withdrawing said veneers from said solution and storing them in an atmosphere at a temperature in the range from room temperature to about 97C. maintained above the equilibrium relative humidity of said aqueous solution for a time sufficient to permit solution retained by each veneer to diffuse throughout its thickness, and

iv. drying said veneers, and

v. removing any excess agent at the surface of said veneers.

14. The process as defined in claim 13 in which said veneers are presoaked to a moisture content of at least about 100 percent oven dried weight.

15. The process as defined in claim 13 in which the fire-retardant agent is a water soluble organic salt, a water soluble inorganic salt, or mixtures of two or more of said salts.

16. The process as defined in claim 14 in which the fire-retardant agent is an inorganic salt having high solubility in water and a solubility which is highly temperature dependent, or a mixture of two or more of such" salts.

17. The process as defined in claim 13 in which said immersion in the solution of fire-retardant agent is for a time sufficient to achieve an uptake of said flameretardant agent of at least pounds per cubic foot of wood.

18. The process as defined in claim 13 in which said solution of fire-retardant agent is a saturated solution of ammonium sulfate, monoammonium phosphate, diammonium phosphate, sodium borate, borax-boric acid mixtures, ammonium sulfamate, chromated zinc chloride, an ammonium polyphosphate fertilizer, a high phosphate fertilizer, or mixtures thereof.

19. The process as defined in claim 18 in which the veneers are immersed in said solution for a period of from 3 to 15 minutes.

2 0. The process as defined in claim 13in which said solution of fire-retardant agent is a saturated solution of monoammonium phosphate and said veneers are immersed in said solution for a period of from 9 to l2 minutes.

21. The process as defined in claim 14 in which the veneers are stored in an atmosphere of about 100 percent humidity for aperiod of from I to 4 days.

22. In a process for manufacturing fire-retardant plywood from a plurality of Douglas fir veneers, the im provement which consists of subjecting said veneers before adhesive coating, assembly and lamination, to a treatment conducted at ambient pressure which comprises successively: v

i. presoaking said veneers in water at about C. for

a period of 24 to 48 hours,

ii. immersing said veneers in a saturated aqueous solution of monoammonium phosphate at a temperature in the range from about 75C. to about C. for a period of about 9 to 12 minutes, v

iii. withdrawing said veneers from said solution and storing them in an atmosphere maintained above IINITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 840. 388 Dat d October 8, 1974 Tibor Gm Perlus and Walter G. Soroka It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

At Page 1, Col. 1 following "[21] Appl. No. 257, 198" insert Foreign Application Priority Date May 28, 1971 Canada 114, 218-- Signed and sealed this 17th day of June 1975.

(SEAL) Attest:

C. i-IARSIZALL DANN R 'IH C. 2291: 301? Commissioner of Patents Attes'cing Officer and Trademarks po'wso uscokm-oc 60376-P69 GOVINuENT PRINYIuG OFFICE: IQ" 0"66'3" 

1. A METHOD FOR IMPREGNATING WOOD LAMINAE WHICH COMPRISES: SELECTING WOOD LAMINAE HAVING A MINIMUM MOISTURE CONTENT OF ABOUT 70 PERCENT OVEN DRIED WEIGHT, SUBJECTING SAID LAMINAE HAVING A MINIMUM MOISTURE CONTENT OF ABOUT 70 PERCENT OVEN DRIED WEIGHT TO A TREATMENT CONDUCTED AT AMBIENT PRESSURE WHICH COMPRISES SUCCESSIVELY: I. IMMERSING SAID LAMINAE IN A CONCENTRATED AQUEOUS SOLUTION OF FIRE-RETARDANT ENERGY MAINTAINED AT AN ELEVATED TEMPERATURE OF ABOUT 75* TO ABOUT 95*C., FOR A TIME SUFFICIENT TO ACHEIVE A LOADING OF THE LAMINAE WITH SAID FLAME RETARDANT AGENT IN AN AMOUNT OF AT LEAST 2 LBS. PER CUBIC FOOT, II. WITHDRAWING SAID LAMINAE FORM SAID SOLUTION AND STORING THEM IN AN ATMOSPHERE AT A TEMPERATURE IN THE RANGE FROM ROOM TEMPERATURE TO ABOUT 97*C. MAINTAINED ABOVE THE EQUILIBRIUM RELATIVE HUMIDITY OF SAID AQUEOUS SOLUTION FOR A TIME SUFFICIENT TO PERMIT SOLUTION RETAINED BY EACH LAMINA TO DIFFUSE THROUGHOUT ITS THICKNESS, AND III. DRYING SAID LAMINAE.
 2. The method as defined in claim 1 in which the selected wood laminae have a minimum moisture content of about 100 percent oven dried weight.
 3. The method as defined in claim 2 in which the minimum moisture content is attained by pre-soaking said laminae in water at an elevated temperature for a time sufficient to produce the minimum moisture content desired.
 4. The method as defined in claim 1 in which said immersion in the solution of fire-retardant agent is for a time sufficient to achieve an uptake of said flame-retardant agent of at least 5 pounds per cubic foot of wood.
 5. The method as defined in claim 1 in which said fire-retardant agent is a water soluble organic salt, a water soluble inorganic salt, or mixtures of two or more of said salts.
 6. The method as defined in claim 2 in which said fire-retardant agent is an inorganic salt having a high solubility in water and a solubility which is highly temperature dependent, or a mixture of two or more of such salts.
 7. The method as defined in claim 2 in which said solution of fire-retardant agent is a saturated solution of ammonium sulfate, monoammonium phosphate, diammonium phosphate, sodium borate, borax-boric acid mixtures, ammonium sulfamate, chromated zinc chloride, an ammonium polyphosphate fertilizer, a high phosphate fertilizer, or mixtures thereof.
 8. The method as defined in claim 1 in which the laminae are immersed in said solution for a period of from about 3 to 15 minutes.
 9. The method as defined in claim 2 in which said solution of fire-retardant agent is a saturated solution of monoammonium phosphate and said laminae are immersed in said solution for a period of approximately 9 to 12 minutes.
 10. The method as defined in claim 1 in which said laminae are stored in an atmosphere of about 100 percent relative humidity for a period of from 1 to 4 days.
 11. A method for impregnating laminae of Douglas fir which comprises in successive steps, presoaking said laminae in water at about 80*C. for a period of from 24 to 48 hours, immersing said laminae in a saturated aqueous solution of monoammonium phosphate maintained at a temperature in the range from about 75*C. to about 95*C. for a period of about 9 to 12 minutes, withdrawing said laminae from said solution and storing them in an atmosphere maintained above the equilibrium relative humidity of said aqueous solution at a temperature in the range from about 40*C. to about 70*C. for a period from 1 to 4 days, and drying said laminae.
 12. In a process for manufacturing fire-retardant plywood from a plurality of wood veneers, the improvement which consists of subjecting veneers selected to have a minimum moisture content of about 70 percent oven dried weight before adhesive coating, assembly and lamination, to a treatment conducted at ambient pressure which comprises successively: i. immersing said veneers in a concentrated aqueous solution of fire-retardant agent maintained at an elevated temperature of about 75* to 95*C., for a time sufficient to achieve a loading of the lamina with said flame-retardant agent in an amount of at least 2 lbs. per cubic foot, ii. withdrawing said veneers from said solution and storing them in an atmosphere at a temperature in the range from room temperature to about 97*C. maintained above the equilibrium relative humidity of said aqueous solution for a time sufficient to permit solution retained by each lamina to diffuse throughout its thickness, and iii. drying said veneers.
 13. In a process for manufacturing fire-retardant plywood from a plurality of wood veneers, the improvement which consists of subjecting said veneers before adhesive coating, assembly and lamination to a treatment conducted at ambient pressure which comprises successively: i. presoaking said veneers in water at an elevated temperature for a time sufficient for the veneers to attain a minimum moisture content of about 70 percent oven dried weight, ii. immersing said veneers in a concentrated aqueous solution of fire-retardant agent maintained at an elevated temperature of about 75* to about 95*C., for a time sufficient to achieve a loading of the lamina with said flame-retardant agent in an amount of at least 2 lbs. per cubic foot, iii. withdrawing said veneers from said solution and storing them in an atmosphere at a temperature in the range from room temperature to about 97*C. maintained above the equilibrium relative humidity of said aqueous solution for a time sufficient to permit solution retained by each veneer to diffuse throughout its thickness, and iv. drying said veneers, and v. removing any excess agent at the surface of said veneers.
 14. The process as defined in claim 13 in which said veneers are presoaked to a moisture content of at least about 100 percent oven dried weight.
 15. The process as defined in claim 13 in which the fire-retardant agent is a water soluble organic salt, a water soluble inorganic salt, or mixtures of two or more of said salts.
 16. The process as defined in claim 14 in which the fire-retardant agent is an inorganic salt having high solubility in water and a solubility which is highly temperature dependent, or a mixture of two or more of such salts.
 17. The process as defined in claim 13 in which said immersion in the solution of fire-retardant agent is for a time sufficient to achieve an uptake of said flame-retardant agent of at least 5 pounds per cubic foot of wood.
 18. The process as defined in claim 13 in which said solution of fire-retardant agent is a saturated solution of ammonium sulfate, monoammonium phosphate, diammonium phosphAte, sodium borate, borax-boric acid mixtures, ammonium sulfamate, chromated zinc chloride, an ammonium polyphosphate fertilizer, a high phosphate fertilizer, or mixtures thereof.
 19. The process as defined in claim 18 in which the veneers are immersed in said solution for a period of from 3 to 15 minutes.
 20. The process as defined in claim 13 in which said solution of fire-retardant agent is a saturated solution of monoammonium phosphate and said veneers are immersed in said solution for a period of from 9 to 12 minutes.
 21. The process as defined in claim 14 in which the veneers are stored in an atmosphere of about 100 percent humidity for a period of from 1 to 4 days.
 22. In a process for manufacturing fire-retardant plywood from a plurality of Douglas fir veneers, the improvement which consists of subjecting said veneers before adhesive coating, assembly and lamination, to a treatment conducted at ambient pressure which comprises successively: i. presoaking said veneers in water at about 80*C. for a period of 24 to 48 hours, ii. immersing said veneers in a saturated aqueous solution of monoammonium phosphate at a temperature in the range from about 75*C. to about 95*C. for a period of about 9 to 12 minutes, iii. withdrawing said veneers from said solution and storing them in an atmosphere maintained above the equilibrium relative humidity of said aqueous solution at a temperature in the range from about 40*C. to about 70*C. for 1 to 4 days, and iv. drying said veneers, and v. removing any excess salt at the surface of said veneers. 